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The relationships between pathogens and hosts are antagonistic interactions. The outcome of these interactions depends on the virulence of the pathogen and on the susceptibility and resistance of the host [2]. Infection is not synonymous with disease since an infection does not always lead to damage to the host. Accordingly, it is important to recognize that the extent of damage during an infection depends on both the potency of the pathogen and the efficiency and mode of the host defense.

Pathogens often produce virulence factors such as adhesins and toxins, which enable them to persist or multiply on or within their host. Capsules of bacteria or fungi such as Cryptococcus neoformans are defensive pathogenicity factors. Numerous important pathogenicity mechanisms rely on the secretion of molecules. The most simple secretory apparatus is the type I secretion system, which consists of only three proteins. The so-called main branch of the general secretion pathway is the type II secretion pathway. Via this pathway toxins and hydrolytic enzymes with a discrete conformation can be excreted. It is widely distributed among Pro-teobacteria, where it contributes to pathogenesis in plants as well as in animals [3]. Type III and IV secretion systems are often encoded by pathogenicity islands. Type III secretion systems occur in human, animal, and plant pathogens. The injected proteins alter or inhibit the normal function of the host cell to allow the pathogen to survive in the host environment. Type IV secretion systems are evolu-tionarily related to the conjugal transfer system used for horizontal gene transfer. Several important human pathogens such as Pseudomonas aeruginosa and Legionella pneumophila inject crucial virulence factors by this system [4, 5]. In addition to effector molecules, virulence is also largely influenced by regulatory systems. Many pathogens like P. aeruginosa sense cell densities and regulate specific sets of genes by quorum sensing [6].

Susceptibility to one particular disease and immunity to others is an innate property of a given host species and is governed by a complex number of different factors. Differences in physiology, anatomy, variations in tissue surface receptors,

20.3 Arabidopsis thaliana: A Plant as a Model for Human Disease | 447

age, diet, and stress are important factors in this respect. Leucine-rich repeat (LRR) proteins seem to play a major role as sensors for pathogen invasion in all eukaryote cells. LRRs are versatile binding motifs found in a variety of proteins and are involved in protein-protein interactions. The LRR domain is composed of repeats forming a characteristic solenoid horseshoe structure which provides a scaffold for numerous insertions involved in binding to pathogen-associated molecular patterns and surface receptors. LRRs have been shown to be involved in the host defense systems of both plants (resistance genes) and mammals (Toll-like receptors and nucleotide-binding oligomerization domain proteins), where they sense specific pathogen-associated molecules and activate the innate immune system [7]. Other defense mechanisms are constituted by physical barriers, the complement system, antimicrobial peptides, apoptosis, effector cells of the innate immune system, and the adaptive immune system.

The different model organisms provide species-specific advantages for the analysis of the pathogen and host side of infection. Mammalian models obviously share many basic biological functions with humans, such as the development of organs and the adaptive immune response. However, invertebrate models can have advantages for the study of certain mammalian biological processes. Because of their lower level of complexity, invertebrate models allow dissection of specifically the innate immune system, undisturbed by superimposed effects of the acquired immune system. Furthermore, a smaller set of genes and/or a smaller proportion of noncoding sequences in the genome can make it easier to trace back mutations to the genes responsible for phenotypic alterations of the host. Last but not least, ethical restrictions are less to the fore in nonvertebrate models. In the following sections we compare and highlight the specific features of different models.

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